1. Field of the Invention
The present invention relates to a thyristor, and more particularly relates to an SCR (silicon-controlled rectifier) which is preferably formed as an SCR element in an integrated circuit (IC).
2. Description of the Prior Art
As is widely known, the thyristor is a three-electrode semiconductor device provided with a gate, a cathode and an anode. Once a gating voltage having a predetermined level is applied to the gate of the thyristor, the path between the anode and cathode becomes conductive and remains in a conductive state without further application of the gating voltage. On the other hand, during said conductive state of the thyristor, once the path between the anode and cathode thereof is caused to become non-conductive, the thyristor ceases to conduct and remains in a non-conductive state unless the gating voltage is applied again to the gate.
The above mentioned typical operation of the thyristor is a widely known characteristic function of the thyristor. Many kinds of apparatuses have been proposed and put into practice, which operate by utilizing this characteristic function of thyristor. However, in some apparatuses which utilize the thyristors, it is not always convenient to utilize this characteristic function of the thyristor. For example, in an apparatus which contains a load to be driven or not driven under control of the thyristor, it is preferable not to utilize this original characteristic function of the thyristor, as the load itself is liable to become conductive or non-conductive. In this case, although it is necessary to supply a driving current continuously to the load by way of the thyristor, if once the driving current is stopped from flowing through the load due to the occurrence of a non-conductive state of the load itself, the driving current will not flow through the load and, therefore, will be stopped from flowing through the thyristor. This is because, as previously mentioned, once the driving current is stopped from flowing through the load and also the thyristor, the thyristor will not be conductive and will be in a non-conductive state, unless the gating voltage is applied again to the gate of the thyristor, even though the load itself may be restored to a conductive state. Accordingly, in such an apparatus, it is necessary for the thyristor to become conductive when a predetermined gating voltage is applied to the gate thereof, in the usual manner, and further it is also necessary for the thyristor to remain in said conductive state even though the driving current is momentarily interrupted by an instantaneous non-conductive state created by the load itself. Thus, the thyristor can be kept in a conductive state, even though the driving current momentarily stops flowing through the thyristor.
It may be possible to create such a thyristor, which can be kept in a conductive state, even though the driving current is momentarily interrupted, by combining a typical thyristor component which is commonly available with an additional control circuit which cooperates with the typical thyristor. However, a thyristor which is capable of being kept in a conductive state, but which is not a combination as stated above, has not heretofore been known in the art; such a single thyristor device, formed as a part of an integrated circuit or as a single chip thyristor, would be very desirable. It should be noted that construction of such a thyristor, as a single thyristor device would be very different from the construction of a thyristor which is composed of both the typical thyristor component and the additional control circuit.